The present invention relates to a polymer electrolyte fuel cell to be used for a portable power source, an electric vehicle, a household cogeneration system, and so on.
In a polymer electrolyte fuel cell, a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air are electrochemically reacted to generate an electric energy and a thermal energy simultaneously. FIG. 12 is a schematic sectional view that shows a basic structure of a conventional polymer electrolyte fuel cell.
A unit cell 101, which is a basic structure in a conventional polymer electrolyte fuel cell 300, mainly comprises a polymer electrolyte membrane 111 which selectively transfers cation (hydrogen ion), and a pair of electrodes (anode and cathode) 112, 113 disposed on both sides of the polymer electrolyte membrane 111. An anode 112 and a cathode 113 are composed of a catalyst layer comprising a mixture of a carbon powder carrying an electrode catalyst (platinum for example) and a hydrogen ion conductive polymer electrolyte, and a gas diffusion layer comprising for example a water repellent treated carbon paper, formed on outside of the catalyst layer and having both gas permeability and electron conductivity.
Then, gas sealing members 114 such as gasket are disposed to sandwich the polymer electrolyte membrane 111, at the peripheries of the anode 112 and the cathode 113 to avoid leaking of the fuel gas and the oxidant gas to outside and mixing of the fuel gas and the oxidant gas together. The sealing member 114 is integrated with the anode 112, the cathode 113, and the polymer electrolyte membrane 111 to form a membrane-electrode assembly (MEA hereafter). Outside the MEA, an anode side separator 116 and a cathode side separator 117 having electric conductivity are disposed to mechanically fix MEA and electrically connect adjacent MEAs each other in series.
In portions of the anode side separator 116 and the cathode side separator 117 contacting with the MEA, gas flow paths 118, 120 are formed to supply reactant gases (fuel gas and oxidant gas) to the anode 112 and the cathode 113 respectively, and to remove a generated gas and an excess gas. Although the gas flow paths 118, 120 may be provided separately from the anode side separator plate 116 and the cathode side separator plate 117, generally, grooves are formed on the surfaces of the anode side separator plate 116 and the cathode side separator plate 117 to serve as the gas flow paths, as shown in FIG. 12.
These MEA, the anode side separator 116, and the cathode side separator 117 form the unit cell 101. Although the unit cell 101 is used alone in some cases, in order to obtain sufficient cell output, the MEA is stacked alternately with the anode side separator 116 and the cathode side separator 117 interposing a cooling unit (not shown) to form a stack (i.e., 10-200 unit cells 101 are stacked). Then, generally, the stack is sandwiched by end plates, with current collector plates and insulating plates interposed in between, and fastened together with fastening bolts and nuts from both sides thereof, thereby making the polymer electrolyte fuel cell 300.
In such a conventional polymer electrolyte fuel cell 300, the anode side separator plate 116 and the cathode side separator plate 117 are formed of carbon flat plates, and on sides contacting the anode 112 and the cathode 113, the gas flow paths 118, 120 for supplying fuel gas or oxidant gas are respectively formed, and on the other side, coolant water flow paths 119, 121 for circulating coolant water are formed. Then, generally, a main surface at a center part of the anode side separator 116 and the cathode side separator 117 where the gas flow path is formed, and a peripheral edge portion at a surrounding part contacting one side of gaskets sandwiching the polymer electrolyte membrane 111 form the same plane without any elevation change.
Here, in the polymer electrolyte fuel cell 300 as mentioned above, the MEA is sandwiched by the anode side separator plate 116 and the cathode side separator plate 117, keeping an appropriate pressure in between the three; the polymer electrolyte membrane 111, the anode 112, and the cathode 113. This is because, it is desired to bring a gas diffusion layer of the anode 112 into contact with the anode side separator plate 116, and to bring a gas diffusion layer of the cathode 113 into contact with the cathode side separator plate 117.
It is also desired to compress a pair of gaskets 114 sandwiching peripheral edge portions of the polymer electrolyte membrane 111 by the anode side separator plate 116 and the cathode side separator plate 117 to seal a peripheral edge portion of the MEA. At this time, the compression degree {i.e., a thickness of a gasket which decreases due to the compression (a difference between a thickness of a gasket before the compression and a thickness of a gasket after the compression)} defines a contact force between a gas diffusion layer of the anode 112 and the anode side separator plate 116, and a contact force between a gas diffusion layer of the cathode 113 and the cathode side separator plate 117.
However, in the anode side separator plate 116 and the cathode side separator plate 117, when a part (main surface) contacting the anode 112 or the cathode 113 and a part (peripheral edge portion) contacting the gasket 114 are in the same plane, like above, in the case the main surface becomes thinner than the peripheral edge portion due to manufacturing tolerance, a sufficient contact between the gas diffusion layer and the anode side separator plate 116 or the cathode side separator plate 117 can not be secured, thereby increasing electric resistance between them. This frequently happens when the gas diffusion layer is formed of a soft material such as a carbon paper. Therefore, in order to suppress such increase in electric resistance, a contact force between the main surface and the gas diffusion layer must be made stronger, by further increasing the compression degree of the gasket 114.
On the other hand, in the case an average thickness of the main surface of the anode side separator plate 116 or the cathode side separator plate 117 is excessively thicker than an average thickness of the peripheral edge portion, when the gasket 114 is compressed such that an appropriate seal ability is obtained, the main surface of the anode side separator plate 116 or the cathode side separator plate 117 excessively compresses the gas diffusion layer. In such case, gas diffusivity is disturbed to create problems: pressure loss of the unit cell 101 is increased, and the gas diffusion layer is buckled to damage the MEA. Furthermore, because the gas diffusion layer enters into the gas flow path 118, 120 formed on the main surface, and closes the gas flow path, an increase in pressure loss of the gas flow path 118, 120 raises possibility of uneven distribution of the reactant gas to the gas flow path 118, 120.
Furthermore, because the MEA, the anode side separator plate 116, and the cathode side separator plate 117 are clamped, the peripheral edge portions of the anode side separator plate 116 and the cathode side separator plate 117 are bent by the clamping force, thereby making contact with the MEA. In the case the anode side separator plate 116 and the cathode side separator plate 117 are planer as in the above, this bent will add a partial load on a peripheral edge portion of the gas diffusion layer, leading to problems in that the gas diffusion layer gives damage on the polymer electrolyte membrane 111, creating a pinhole on the polymer electrolyte membrane 111.
The present invention was made in view of the foregoing problems, and an object of the present invention is to provide a polymer electrolyte fuel cell in which a sealing effect can be displayed without gas leak by compressing a gasket sufficiently, increase in electrical resistance (contact resistance) between a gas diffusion layer of an anode and an anode side separator plate, and between a gas diffusion layer of a cathode and a cathode side separator plate can be suppressed, and increase in pressure loss due to close of a gas flow path and damage to a polymer electrolyte membrane by the gas diffusion layer can be avoided. Further, another object of the present invention is to provide a separator plate that can easily and reliably embody the above mentioned polymer electrolyte fuel cell.